# std::integral_constant

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Defined in header `<type_traits>` |  |
template< class T, T v > struct integral_constant; |  | (since C++11)

`std::integral_constant` wraps a static constant of specified type. It is the base class for the C++ type traits.

If the program adds specializations for `std::integral_constant`, the behavior is undefined.

### Helper alias templates

A helper alias template `std::bool_constant` is defined for the common case where `T` is bool.

template< bool B > using bool_constant = integral_constant<bool, B>; |  | (since C++17)

### Specializations

Two typedefs for the common case where `T` is bool are provided:

Defined in header `<type_traits>`
Name | Definition
`true_type` | std::integral_constant<bool, true>
`false_type` | std::integral_constant<bool, false>

### Member types

Name | Definition
`value_type` | T
`type` | std::integral_constant<T, v>

### Member constants

Name | Value
constexpr T value [static] | v (public static member constant)

### Member functions

operator value_type | returns the wrapped value (public member function)
operator() (C++14) | returns the wrapped value (public member function)

## std::integral_constant::operator value_type

constexpr operator value_type() const noexcept; |  |

Conversion function. Returns the wrapped value.

## std::integral_constant::operator()

constexpr value_type operator()() const noexcept; |  | (since C++14)

Returns the wrapped value. This function enables `std::integral_constant` to serve as a source of compile-time function objects.

### Possible implementation

    template<class T, T v>
    struct integral_constant
    {
        static constexpr T value = v;
        using value_type = T;
        using type = integral_constant; // using injected-class-name
        constexpr operator value_type() const noexcept { return value; }
        constexpr value_type operator()() const noexcept { return value; } // since c++14
    };

### Notes

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_integral_constant_callable` | `201304L` | (C++14) | `std::integral_constant::operator()`
`__cpp_lib_bool_constant` | `201505L` | (C++17) | `std::bool_constant`

### Example

Run this code

    #include <type_traits>

    using two_t = std::integral_constant<int, 2>;
    using four_t = std::integral_constant<int, 4>;

    static_assert(not std::is_same_v<two_t, four_t>);
    static_assert(two_t::value * 2 == four_t::value, "2*2 != 4");
    static_assert(two_t() << 1 == four_t() >> 0, "2*2 != 4");

    enum class E{ e1, e2 };
    using c1 = std::integral_constant<E, E::e1>;
    using c2 = std::integral_constant<E, E::e2>;
    static_assert(c1::value != E::e2);
    static_assert(c1() == E::e1);
    static_assert(std::is_same_v<c2, c2>);

    int main() {}

### See also

integer_sequence (C++14) | implements compile-time sequence of integers (class template)

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